1. Field of the Invention
The present invention relates to a wafer cleaning bath or tank in which one or more wafers of semiconductor single crystal (hereinafter referred to as "wafers") are substantially vertically immersed batch by batch in a cleaning solution to clean off or remove dirt and residual impurities left on the wafers.
2. Description of the Related Art
In the semiconductor device fabrication, residual impurities on the surface of a wafer affect the performance of fabricated semiconductor devices.
In view of this, the wafer shaping process includes a cleaning step in which wafers are cleaned by use of various cleaning methods. The wafer cleaning methods currently used are generally classified into a physical cleaning method and a chemical cleaning method.
The physical cleaning method includes: brush scrubbing in which a cleaning brush is used to directly remove residual impurities and foreign matters left on the wafer surface; high-pressure jet scrubbing in which a high-pressure fluid jet from a nozzle is directed to a portion of or the entire surface of a wafer to remove impurities; and ultrasonic cleaning in which wafers are immersed in a solution and are agitated by ultrasonic energy to remove residual impurities on the wafers.
On the other hand, the chemical cleaning method includes: chemical decomposition cleaning in which various chemicals and enzymes are used to chemically decompose impurities and foreign matters on the wafer surface. The physical cleaning method and the chemical cleaning method may be used in combination with each other.
In the semiconductor device fabrication art, the wafer diameter has been steadily increasing to cope with the tendency of semiconductor devices toward higher scale integration. Accordingly, a conventional cleaning method in which a plurality of wafers are received at one time in the same carrier cassette requires a great deal of labor.
Furthermore, if the wafers are transported while they are received in the carrier cassette, such transportation will encounter a problem that the wafers are contaminated with particles and the like adhering thereto due to contact between the wafers and the carrier cassette.
With the foregoing difficulties in view, the present inventors have proposed a cassette-less automatic wafer cleaning equipment in which wafers are substantially vertically supported one at a time and cleaned by immersing them one by one in a cleaning solution in each of successive cleaning baths or vessels (Japanese Patent Laid-open Publication No. 5-36667). Also proposed by the present inventors have been a wafer cleaning tank which is capable of immersing one wafer at a time in a cleaning solution while supporting the wafer stably in a substantially vertical position (Japanese Patent Laid-open Publication No. 5-36664).
The proposed cassette-less automatic wafer cleaning equipment has received favorable appreciation for its ability of providing a considerable reduction of labor as compared to the aforesaid conventional cleaning method in which a carrier cassette is used to receive a plurality of wafers, and solving the wafer-contamination problem caused by the carrier cassette.
However, the cassette-less automatic wafer cleaning equipment still has a problem that since the wafers are cleaned one by one or on the sheet-by-sheet basis, a further improvement in the productivity cannot be expected. In addition, for this cleaning equipment, it is extremely difficult to deal with the cleaning of wafers of different diameters because a high positioning accuracy required for setting and handling of the wafers renders the cleaning equipment complicated in construction.
Through investigations started with a view to improving the productivity of the cassette-less automatic wafer cleaning equipment, the present inventors have succeeded in devising a unique wafer supporting mechanism which is capable of supporting a plurality of wafers at one time in a cleaning tank, thereby improving the productivity of a cleaning equipment in an unrestricted manner, and which employs a three-point support structure enabling the cleaning of wafers of different diameters.